A slab, which is a steel semi-finished product, a thick sheet produced using the slab, and the like are conveyed on a production line composed of a plurality of rolls during the production process thereof. At this time, shape measurement using what is called a light-section method is performed in order to measure the surface height of a rigid body such as the slab or the thick sheet. However, when a rigid body such as a slab or a thick sheet is conveyed on a production line, there has been a problem that the fluctuation in the surface height derived from the vertical movement and rotation of the rigid body (hereinafter, referred to as “disturbance”) is superimposed on the measured surface height, and the true surface height cannot be measured.
To address the problem mentioned above, a technology shown in Patent Literature 1 below proposes forming, in addition to a light-section line for the original shape measurement formed in the width direction of a rigid body to be measured, another light-section line in a direction oblique to the light-section line mentioned above (directions that are not mutually parallel). In the technology, the measurement of the same point of the rigid body to be measured, which is originally supposed to have the same surface height, is performed twice for each of a plurality of points of different longitudinal-direction positions and different width-direction positions. After that, the magnitude of disturbance (vertical movement and rotation) by which the surface heights of the plurality of points mentioned above coincide most favorably is obtained by optimization calculation, and the effect of disturbance is removed from the measurement result.